There is a strong demand from automobile manufacturers and other users for a one-side electrogalvanized steel strip of which the other surface not electrogalvanized is beautiful in external appearance, excellent in corrosion resistance after painting, and free from painting irregularities and defective adherence of paint.
A one-side electrogalvanized steel strip is conventionally manufactured as follows: continuously causing a steel strip to travel through an electrogalvanizing bath in an electrogalvanizing tank; arranging at least one electrode plate in the electrogalvanizing bath in the electrogalvanizing tank in parallel to a one-side surface to be electrogalvanized of the travelling steel strip; and causing an electric current to flow between the at least one electrode plate and the travelling steel strip through the electrogalvanizing bath to electrogalvanize the one-side surface of the travelling steel strip.
However, the above-mentioned conventional manufacturing method has a problem in that yellowish contamination is produced on the other surface not electrogalvanized of the steel strip which has passed through the electrogalvanizing bath in the electrogalvanizing tank. A main cause of the production of such yellowish contamination is that the other surface not galvanized of the steel strip which has passed through the electrogalvanizing tank is oxidized by an electrogalvanizing solution deposited on the other surface. Particularly, the above-mentioned contamination on the other surface not electrogalvanized of the steel strip becomes more serious when the one-side electrogalvanized steel strip is heated to alloy an electrogalvanized layer formed on the one-side surface of the steel strip into an iron-zinc alloy layer.
The presence of the above-mentioned contamination on the other surface not electrogalvanized of the steel strip causes the following problems:
(1) The other surface not electrogalvanized of the steel strip loses the gloss thereof, and thus the external appearance of the other surface worsens.
(2) When a phosphate treatment is applied to the other surface not electrogalvanized of the steel strip, a phosphate film having dense crystals cannot be formed on the other side.
(3) When painting is applied onto the phosphate film formed on the other surface not electrogalvanized of the steel strip, the other surface shows a degraded corrosion resistance after painting, and painting irregularities and defective adherence of paint are caused.
As a method for solving the above-mentioned problems, a method for manufacturing a one-side electrogalvanized steel strip is disclosed in Japanese Patent Provisional Publication No. 55-79,878 dated June 16, 1980, which comprises:
continuously causing a steel strip to travel through an electrogalvanizing bath in an electrogalvanizing tank; arranging at least one electrode plate in said electrogalvanizing bath in said electrogalvanizing tank in parallel to a one-side surface to be electrogalvanized of said travelling steel strip; causing an electric current to flow between said at least one electrode plate and said travelling steel strip through said electrogalvanizing bath to electrogalvanize said one-side surface of said travelling steel strip; arranging at least one spray nozzle toward the other surface not electrogalvanized of said travelling steel strip which has passed through said galvanizing tank; ejecting water from said at least one spray nozzle onto said other surface of said travelling steel strip to wet said other surface of said travelling steel strip; arranging at least one brush roll in contact with said other surface not electrogalvanized of said travelling steel strip; and rotating said at least one brush roll to eliminate contamination caused by the deposition of an electrogalvanizing solution on said other surface of said travelling steel strip (hereinafter referred to as the "prior art").
However, the above-mentioned prior art has the following problems:
(1) Elimination of contamination produced on the other surface not electrogalvanized of the one-side electrogalvanized steel strip is insufficient. Therefore, the external appearance of the other surface cannot be made sufficiently beautiful. In addition, when applying a phosphate treatment to the other surface not electrogalvanized of the steel strip, crystals of a phosphate film formed on the other surface cannot be made sufficiently dense. Furthermore, when applying painting onto the phosphate film formed on the other surface not galvanized of the steel strip, corrosion resistance after painting of the other surface is insufficient, and painting irregularities and defective paint adherence cannot be prevented from occurring on the other surface.
(2) Contamination produced on the other surface not galvanized of the travelling steel strip cannot be sufficiently eliminated unless the rotating brush roll is strongly pushed to the other surface. This leads to a serious wear of the brush roll.
Under such circumstances, there is a strong demand for the development of a method for manufacturing a one-side electrogalvanized steel strip, which permits sufficient elimination of contamination produced on the other surface not electrogalvanized of the one-side electrogalvanized steel strip, permits formation of a phosphate film having dense crystals on the other surface not electrogalvanized when applying a phosphate treatment to the other surface, and when applying painting onto the phosphate film formed on the other surface not galvanized, permits impartment of a high corrosion resistance after painting to the other surface and prevention of painting irregularities and defective paint adherence from occurring on the other surface, but such a method has not as yet been proposed.